When making reservations to stay at a non-Disney hotel you should ask a lot of questions about their transportation to the parks. Many of the hotels do not have their own shuttles. Instead they contract with local bus services that go around from hotel to hotel picking up passengers before heading to the parks. They also sometimes go from park to park dropping off passengers. This can make for a long ride. Many of the hotel shuttles do not operate continuously, meaning if you miss your shuttle the next one may not arrive for hours.
If that isn't bad enough, the non-Disney shuttles are not permitted to use the Disney bus stops. They must load and unload at designated spots out in the public parking lots where there are no benches and no shade. And, as Disneycrazymom pointed out, they cannot go directly to MK but must load and unload at the TTC parking lot.
If you are walking distance from Marketplace you can catch a Disney bus there. But they don't go directly to the parks from Downtown Disney. You will still have to transfer at a the TTC.